Relation between RPM, EGT and Mixture

[Chrisgoesflying]

There are plenty of articles about this but I wanted to launch this topic here (probably for 100th time) as it seems there are so many different opinions. Can someone on here, in plain English, but completely scientific, explain how leaning the mixture influences RPM and EGT and why it's all linked (assuming fixed pitch prop, your typical Cessna 172/Piper Cherokee). I'm a complete klutz when it comes to any of this stuff.

Once this is explained, please also answer the following question (and I know, opinions will differ here). How do you lean properly?

Quick background: I always lean, without really knowing what I'm doing when doing so. I've had three different instructors since I started flying and their opinions ranged from "leaning is important but it's too early for you to worry about it" (that was my first instructor and I just started flying), to "just pull out the mixture in cruise about one thump long and you're good", all the way to "don't worry about leaning". I do worry about leaning, especially as I own my plane. I want to keep the engine in good shape and not burn more fuel than I have to. The way I lean, I get about 8gph in a Cherokee 140 with the 160 HP upgrade. However, I tend to just lean by gut feel, kind of like my second instructor.

On a recent loooooong x-country flight, I got in cruise, set the rpm to 2,400 and then started leaning about a thumb's length. A little later, I noticed the rpm went down to 2,300 rpm. I pushed the throttle in to get back to 2,400 rpm and a little later it again went down to 2,300 rpm. I then pulled the mixture a little further and rpm went up. Pulled the mixture even more and rpm increased again, going to almost 2,500 rpm. Then I looked at the EGT and noticed that the needle is to the right of the star (I have a little star on the EGT gauge - I assume that's the "red line") so I pushed the mixture in a tad, EGT when back down to the left of the star and rpm settled at right around 2,400 rpm. Didn't touch anything afterwards but rpm and EGT stayed at the same spot for the remainder of the flight, about 3 or so hours.

As it was a long X-Country, I was fairly high at about 7,500 ft. Normally, when I fly shorter X-Countries I'm lower than that. Also, OAT was about 15 C on the ground. At least with this plane, I've always flown in MUCH colder temps like -10 C on the ground.

Someone please explain this to me and the proper procedure for best long term engine health and best performance (speed, fuel burn) on my trips.

 

[Martin Pauly]

Can someone on here, in plain English, but completely scientific, explain how leaning the mixture influences RPM and EGT and why it's all linked (assuming fixed pitch prop, your typical Cessna 172/Piper Cherokee).

I explain how those things are connected in this video. For the fixed pitch prop, I'l add that more power means more RPM.

If you want to expand your understanding from how these things are related to why these things are related, the online version of the former APS Class (Advanced Pilot Seminar) covers this in a lot more detail:
https://www.advancedpilot.com/onlinecourse.html
It's not free, but it's great stuff. You asked for "completely scientific", and this course delivers. Sneak peak: A big part of this is the different speeds of the flame front during combustion as a function of how rich or lean the mixture is.

- Martin

 

[Martin Pauly]

Chris,

I want to add I applaud you for asking questions about leaning. Sadly, most flight instructors know very little about leaning and engine management themselves, and therefore merely pass on some ambiguous vague guidance ("sometimes we do this, sometimes we do that") or old-wives-tales ("never lean below 5000 feet"). When you hear something like this, ask: "Why?". The answer (or lack thereof) will tell you a lot about the value of the guidance you have received. Your engine and your pocket book will thank you.

- Martin

 

[AR797]

Chris,

I want to add I applaud you for asking questions about leaning. Sadly, most flight instructors know very little about leaning and engine management themselves, and therefore merely pass on some ambiguous vague guidance ("sometimes we do this, sometimes we do that") or old-wives-tales ("never lean below 5000 feet"). When you hear something like this, ask: "Why?". The answer (or lack thereof) will tell you a lot about the value of the guidance you have received. Your engine and your pocket book will thank you.

- Martin

I cannot agree less..spot on. first hand experience… don’t leave before 3000feet and cruise level… asked why….silence..

 

[Domenick]

I fly/own a Warrior II (-161). I have a 4-cyl CHT-only engine monitor. No EGT, so I do not lean by exhaust gas temperature. I lean according to Lycoming's instructions--lean to engine stumble, enrichen to even running.

During taxi, I lean to the point where advancing the throttle will cause the engine to stumble. This keeps the plugs from fouling and prevents takeoff in an excessively leaned condition.

Typically, I burn 6.5-7.5 gph depending on what I am doing. Just burning holes in the local sky, throttled back to best economy with some pattern work is always less than 7 gph.

Some years back I flew a 6600 nm continental tour yielding 7.4 gph.

My takeoff mixture setting is generally close to full, and I will gently lean as I climb to cruise, where I lean as described above. Takeoffs from high altitude airports (however you define that) is somewhat rich of best rpm.

 

[Martin Pauly]

For anyone who would like to get a better understanding, I'll be presenting the mixture & LOP topic at Sun 'n Fun and AirVenture again this year.

The Sun 'n Fun forums are on Wednesday (April 6) and Thursday (April 7) at 10:00 AM in the CFAA-15 classroom.

A forum schedule for AirVenture has not been published yet.

- Martin

 

[Tantalum]

Thanks Martin. It really is a shame that the lion's share of instructors don't do more to teach what 'mixture' actually does. They rely on rote memorization only. Pilots should understand what's happening in their engines

 

[airdale]

https://www.lycoming.com/content/leaning-lycoming-engines
https://www.lycoming.com/sites/default/files/Fuel Mixture Leaning Procedures.pdf
https://www.lycoming.com/sites/default/files/SSP-475.pdf

 

[FPK1]

https://www.lycoming.com/content/leaning-lycoming-engines
https://www.lycoming.com/sites/default/files/Fuel Mixture Leaning Procedures.pdf
https://www.lycoming.com/sites/default/files/SSP-475.pdf

The wording from this document is exactly what's written in my 2022 AFM with IO390 engine...

 

[Star Keeper]

So I have a related question. Is EGT or CHT gauges required to properly lean an engine? I hear people typically referencing these things when talking about leaning out but the aircraft I fly has an EGT gauge that is inop. Is there a risk of overheating the plane by running too lean?

 

[jesse]

So I have a related question. Is EGT or CHT gauges required to properly lean an engine? I hear people typically referencing these things when talking about leaning out but the aircraft I fly has an EGT gauge that is inop. Is there a risk of overheating the plane by running too lean?

There is no such a thing as "overheating the plane by running too lean". If you run too lean, the engine will quit, and when the engine quits it no longer products heat. More appropriately, would be to say that you can "overheat an engine if you run it at an inappropriate mixture/power setting". Generally speaking, you'll be most likely to overheat a cylinder while in a climb somewhere slightly rich of peak/at peak. The moment you get LOP, things start cooling down.

The "star" on your EGT gauge doesn't mean a damn thing. Ignore it. Pretend it's not there. Honestly, the single cylinder EGT gauge serves no useful purpose. A paperweight on a desk is more useful than a single cylinder EGT gauge on your basic engine. Throw the damn thing away. Shoot it with a shotgun. Do whatever the hell you need to do to not take that into consideration when you're setting your mixture. Given how you have no idea what the hell is happening on the other cylinders, its useless.

Assuming we're talking about a normally aspirated engine, lean it till its rough, and enrichen it to the point to where it runs smoothly and not a touch more. You'll be right where you want to be (which may or may not be LOP, depends on whether the fuel system is balanced enough to keep all the cylinders relatively equal. In most carbed airplanes, it isn't).

If we're talking turbocharged, then, it really starts to depend on your engine/installation...Which I'd need to know more about.

 

[Martin Pauly]

So I have a related question. Is EGT or CHT gauges required to properly lean an engine? I hear people typically referencing these things when talking about leaning out but the aircraft I fly has an EGT gauge that is inop. Is there a risk of overheating the plane by running too lean?

It depends on how much power the engine is making. If you are operating at 65% power or less, leaning will not damage your engine.

The scenario where leaning without good instrumentation can be bad is when you are flying at high power, say full throttle at 2,000 MSL on a cold day. If you pull the mixture knob back until you feel roughness, then forward a tad bit - like most of us learned to lean - there is no way of telling for sure if you are LOP, ROP or at peak unless you have the proper instrumentation. For all I know, half of your cylinders could be LOP and the others ROP. And if any cylinders in this situation are slightly ROP, they will develop very high internal cylinder pressures (ICP) and cylinder head temperatures (CHT), which is bad for the engine.

There is no such a thing as "overheating the plane by running too lean".

Let's be careful here. I would say it this way: At high power output, you CAN overheat the engine by running somewhat lean, but not lean enough.

- Martin

 

[jesse]

There is no such a thing as "overheating the plane by running too lean".

Let's be careful here. I would say it this way: At high power output, you CAN overheat the engine by running somewhat lean, but not lean enough.

What we're really concerned about is the internal cylinder pressures, which will top out at around 40 degrees ROP. Notice how the position of that involves the word "rich". Therefore, given how you need to be RICH of peak to be in that profile, it certainly isn't possible because you were "too lean". Perhaps if you're "not rich enough".

I suppose your statement might make sense if you referred to "40 degrees ROP" as "-40 degrees LOP"...but...anyone that thinks that way is probably a serial killer or something.

Given that the OP is talking about a four cylinder normally aspirated aircraft engine with limited engine instrumentation - he's completely overthinking it. Needs to not worry about it. Lean to rough. Rich to where it sounds smooth/happy. Fly and enjoy it. Rental aircraft with these engines have been leaned this way for decades and have engines that go thousands of hours over TBO.

 

[Chrisgoesflying]

@jesse ... I see you're in Lincoln. I just flew to Omaha today so almost neighbors for the next two days before heading further north to go back home to Canada

On the flight from Arkansas to Omaha today, I leaned and was probably overthinking the whole EGT thing again. I kept pulling the mixture and the EGT went pretty much all the way to the right. Engine was purring without a stumble. When I saw the EGT needle all the way on the right, I was afraid to keep leaning it even more so I pushed the mixture back in a tad and EGT came down to around the location where the star (which I should be ignoring?) is located.

Long story short, you're saying I just keep pulling on the mixture knob until the engine stumbles, and then push in until smooth, completely ignoring the EGT gauge? If the EGT gauge is that useless, why did they put it in the plane?

 

[jesse]

@jesseOn the flight from Arkansas to Omaha today, I leaned and was probably overthinking the whole EGT thing again. I kept pulling the mixture and the EGT went pretty much all the way to the right. Engine was purring without a stumble. When I saw the EGT needle all the way on the right, I was afraid to keep leaning it even more so I pushed the mixture back in a tad and EGT came down to around the location where the star (which I should be ignoring?) is located.

Long story short, you're saying I just keep pulling on the mixture knob until the engine stumbles, and then push in until smooth, completely ignoring the EGT gauge? If the EGT gauge is that useless, why did they put it in the plane?

Long story short. Yes. That is exactly what I am saying there is precisely 0% chance you would damage your engine by "leaning till smooth, enrichen till rich" during cruise on your flight from Arkansas to Omaha today with a normally aspirated engine from Lycoming or Continental. If anything, you'd be running it at the correct spot and will decrease the likelihood of an engine issue.

EGTs are relative temperatures. The "right" temperature for one airplane at one given moment, will be completely different in a different moment, or in a different airplane. EGT probes are also installed in different locations on different airplanes which means its never possible to say a specific temperature is the correct temperature. As a result, we find the "peak temperature" then we make adjustments relative to that.

So why am I not advising you to find the "peak temperature" then lean relative to that? Well, it's because you have no idea what the EGT is on the other cylinders, and likely all the cylinders peak at different moments. With an engine monitor that lets you see the EGT of every cylinder, you are then able to use the gauge in an intelligent manner. Without the ability to see all the cylinders..its pretty useless..

Why did they only install one gauge? Maybe a previous owner really wanted a gauge but didn't realize a single probe is pretty useless? More likely...its OEM and the OEM installed it in some model year revision so that they could claim they "upgraded" the airplane.

Anyhow..an engine monitor that can monitor all cylinders is very useful and is something you can use to help you lean. What you'll find though...is that spot the engine monitor tells you is the perfect spot...is the same spot you'll be at if you lean till rough then enrichen until smooth. Most of the "value" I get from these multi-cylinder monitors has more to do with monitoring overall engine behavior than it does helping me lean. One can often times spot an issue in the ignition system very early for example by watching the EGTs during mag checks.

The situation changes dramatically once we start talking about turbocharged engines. On some of them, it's possible to grenade the engine within seconds if you put the levers in the wrong spot.

@jesse
I just flew to Omaha today so almost neighbors for the next two days before heading further north to go back home to Canada

Nice! Definitely reach out if you run into any hiccups, happy to help if I can. I'd offer to swing over to Omaha to meetup, but am pretty busy until this weekend. If by some chance you are still around this weekend let me know!

 

[Martin Pauly]

What we're really concerned about is the internal cylinder pressures, which will top out at around 40 degrees ROP. Notice how the position of that involves the word "rich". Therefore, given how you need to be RICH of peak to be in that profile, it certainly isn't possible because you were "too lean". Perhaps if you're "not rich enough".

I agree with all of that, Jesse.

What I was trying to say is that without proper instrumentation, there is no way of telling whether you are actually lean of peak or not by the time the engine starts running rough. This part of the exchange started with the question “Is EGT or CHT gauges required to properly lean an engine?”, and my argument remains that if you are leaning without CHT and EGT gauges, you cannot tell how far rich or lean of peak you are operating, and at high power output that can bite you. You might be at 40 deg ROP on one cylinder, which you correctly identified as a bad place (at high power output) - without knowing you are there.

- Martin

 

[Tantalum]

So I have a related question. Is EGT or CHT gauges required to properly lean an engine? I hear people typically referencing these things when talking about leaning out but the aircraft I fly has an EGT gauge that is inop. Is there a risk of overheating the plane by running too lean?

The EGT is a better and more immediate indicator of the combustion process in your engine.. as you pull the mixture back from full rich you will see the EGT rise. When it's at its highest level then if you keep pulling the mixture back you are basically now in a "lean of peak" operation

Engines really aren't designed to run at peak, it is too hot and damages components. In many power plants you also aren't able to lean past peak, the engine will start to run rough. If you have a well balanced engine and/or good injectors then you can get past peak. Some people claim they can do it in their carbureted engines, I am highly skeptical of that

CHT is important because in the most basic sense you don't want to make certain parts of the engine too hot, but there is a delay in CHT changes with EGT.. so I usually initially set with EGT and then fine tune by looking at the CHT over the span of 10 to 15 minutes. If you are on the rich side of peak then adding mixture will cool the engine, if you are on the lean side of peak then leaning further will cool the engine down

When I fly I look for 380 or lower CHT and around 1400 EGT

Depending on the plane that sometimes means I am on the rich of peak side of things other times I'm able to get on the lean of peak side (fuel gami injected well balanced)



Personally I like to stay len of peak if able, to me it's a more efficient use of the engine and components. Seems ridiculous to keep an engine cool by basically just throwing gas at it

 

[Star Keeper]

The fear that was instilled (possibly incorrectly) in me by previous instructors is fowling of the plugs by being too rich. Thats why we lean out on taxi. If I'm flying a plane with no working EGT should I simply lean to rough and then back? I'm also working with mythical information from my automotive knowledge of being too lean being bad for the engine because it runs too hot.

 

[Tantalum]

too lean being bad for the engine because it runs too hot.

check this out

The fear that was instilled (possibly incorrectly) in me by previous instructors is fowling of the plugs by being too rich. Thats why we lean out on taxi.

this is not incorrect, I did my IR in a plane owned by the instructor, his rule was the plane should be so lean on taxi that any attempt to rev over 1,500 RPM would kill the engine. So that's basically what I do now, it barely stays alive for taxi

If I'm flying a plane with no working EGT should I simply lean to rough and then back?

that's the old tried and true method.. keep slowly leaning until the engine sounds funny / runs rough, then give the mixture a few turns rich, or about half an inch to inch rich if not a vernier mixture

 

[farmrjohn]

[QUOTE="Tantalum, post: 3226008, member: 30125"
If you have a well balanced engine and/or good injectors then you can get past peak. Some people claim they can do it in their carbureted engines, I am highly skeptical of that

[/QUOTE]

What are these "injectors" you refer to, asks the owner of an O200

 

[21541803]

Engines really aren't designed to run at peak, it is too hot and damages components.

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

1. LEANING TO EXHAUST GAS TEMPERATURE GAGE. a. Normally aspirated engines with fuel injectors or uncompensated carburetors. (1) Maximum Power Cruise (approximately 75% power) – Never lean beyond 150°F on rich side of peak EGT unless aircraft operators manual shows otherwise. Monitor cylinder head temperatures. (2) Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.

 

[Tantalum]

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

1. LEANING TO EXHAUST GAS TEMPERATURE GAGE. a. Normally aspirated engines with fuel injectors or uncompensated carburetors. (1) Maximum Power Cruise (approximately 75% power) – Never lean beyond 150°F on rich side of peak EGT unless aircraft operators manual shows otherwise. Monitor cylinder head temperatures. (2) Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.

In general terms for someone asking about basic concepts who might be new to aviation it's good rule of thumb not to hang out at peak

For NA engines at altitude they're generally going to be at 75% or well below so there are less risks RE leaning

 

[William Pete Hodges]

Hopefully you can download the attached file. It explains how all these variables are related in one page. It is not an in-depth discussion, but it is a start.

Have Fun! Fly SAFE! Pete

 

[Jeff767]

https://www.lycoming.com/sites/default/files/O-HO-IO-HIO-AIO & TIO-360 Oper Manual 60297-12.pdf

1. LEANING TO EXHAUST GAS TEMPERATURE GAGE. a. Normally aspirated engines with fuel injectors or uncompensated carburetors. (1) Maximum Power Cruise (approximately 75% power) – Never lean beyond 150°F on rich side of peak EGT unless aircraft operators manual shows otherwise. Monitor cylinder head temperatures. (2) Best Economy Cruise (approximately 75% power and below) – Operate at peak EGT.

This is correct information and many aircraft recommend cruise flight at peak EGT. It’s better for the engine to run at peak instead of 60 to 40 degrees ROP which is the highest internal cylinder pressures and CHT. It’s even better if you can get the engine to operate LOP but as others have pointed out this is usually not possible in a carbed engine. The tried and true method in cruise of lean until it stumbles and then richen it a bit puts most engines at peak.
If the engine is not turbo or supercharged there is really only one way to hurt the motor with the mixture control. That’s to lean the engine excessively while under full power during takeoff and climb and induce detonation. Even if leaned excessively you probably need other factors like poor fuel and or a very hot day to detonate the motor. A simple to keep the engine safe in climb and increase power while reducing fuel flow is to note the EGT at 1000 feet on takeoff and lean as you climb to maintain that EGT. That if the engine is properly set up will keep it well out of the potential detonation zone. As I mentioned however it’s really hard to get a standard compression 4 banger to detonate. On the test stand they often need to get the CHT’s near 500 degrees with a very lean mixture plus reduced RPM to cause detonation. The fact this margin is so big is why many engines can be run on auto fuel with lower octane than AvGas.

 

[Tarheelpilot]

This is correct information and many aircraft recommend cruise flight at peak EGT. It’s better for the engine to run at peak instead of 60 to 40 degrees ROP which is the highest internal cylinder pressures and CHT. It’s even better if you can get the engine to operate LOP but as others have pointed out this is usually not possible in a carbed engine. The tried and true method in cruise of lean until it stumbles and then richen it a bit puts most engines at peak.
If the engine is not turbo or supercharged there is really only one way to hurt the motor with the mixture control. That’s to lean the engine excessively while under full power during takeoff and climb and induce detonation. Even if leaned excessively you probably need other factors like poor fuel and or a very hot day to detonate the motor. A simple to keep the engine safe in climb and increase power while reducing fuel flow is to note the EGT at 1000 feet on takeoff and lean as you climb to maintain that EGT. That if the engine is properly set up will keep it well out of the potential detonation zone. As I mentioned however it’s really hard to get a standard compression 4 banger to detonate. On the test stand they often need to get the CHT’s near 500 degrees with a very lean mixture plus reduced RPM to cause detonation. The fact this margin is so big is why many engines can be run on auto fuel with lower octane than AvGas.

I still think one of the worst things ever done was putting numbers on EGT gauges. No need to know the number. Only relative position to peak.

 

[GeorgeC]

https://www.lycoming.com/content/leaning-lycoming-engines
https://www.lycoming.com/sites/default/files/Fuel Mixture Leaning Procedures.pdf
https://www.lycoming.com/sites/default/files/SSP-475.pdf

Specifically, this:

 

[Trogdor]

EGT is not a *real* number. Period. And CHTs are great provided you have a engine monitor that measures all of them (CHTs are a real number and they can be thought of as "stress" on the cylinder).

I highly, highly recommend you YouTube "Savvy Aviation Lean" and go from there. Mike really answers all of your questions.

 

[airdale]

EGT is not a *real* number. Period. ...

Fascinating. Have you discussed this with the Lycoming engineers? And, what is your technical background compared to the background of the Lycoming engineers?

 

[jesse]

Fascinating. Have you discussed this with the Lycoming engineers? And, what is your technical background compared to the background of the Lycoming engineers?

The “correct” EGT value is relative between different Lycoming installations. Lycoming refers to leaning relative to peak, not relative to a specific number. Same thing applies to Continental.

If you’re flying behind a Rotax, things are a little different. For example, you may see an absolute EGT max temperature.

 

[mandm]

I explain how those things are connected in this video. For the fixed pitch prop, I'l add that more power means more RPM.

If you want to expand your understanding from how these things are related to why these things are related, the online version of the former APS Class (Advanced Pilot Seminar) covers this in a lot more detail:
https://www.advancedpilot.com/onlinecourse.html
It's not free, but it's great stuff. You asked for "completely scientific", and this course delivers. Sneak peak: A big part of this is the different speeds of the flame front during combustion as a function of how rich or lean the mixture is.

- Martin

Great video! I watched it and tried it last night, the EGT kept increasing and the mixture was almost all the way lean, I never got the EGT to drop, when I richened the mixture I felt and heard a noticeable power increase. I will try it again later but worries if I cut the engine power all the way.

 

[Martin Pauly]

the EGT kept increasing and the mixture was almost all the way lean, I never got the EGT to drop,

Did the engine run rough, or what made you stop leaning? Not all engines will run LOP smoothly, but I couldn’t tell from your description if that’s the case with yours.

Regards,
Martin

 

[mandm]

Did the engine run rough, or what made you stop leaning? Not all engines will run LOP smoothly, but I couldn’t tell from your description if that’s the case with yours.

Regards,
Martin

What made me stop leaning, fear and inexperience

I’ll try again though.

 

[Trogdor]

Fascinating. Have you discussed this with the Lycoming engineers? And, what is your technical background compared to the background of the Lycoming engineers?

Watch the Mike Busch video. Then come back.

 

[jesse]

What made me stop leaning, fear and inexperience

I’ll try again though.

Sigh..did your instructor never cover this?

 

[sarangan]

Great video! I watched it and tried it last night, the EGT kept increasing and the mixture was almost all the way lean, I never got the EGT to drop, when I richened the mixture I felt and heard a noticeable power increase. I will try it again later but worries if I cut the engine power all the way.

You didn't say whether this was carbureted or fuel injected. With fuel injection and an analyzer, it is easy to lean very accurately. With a carb, it is crapshoot. Ideally, the EGT probe should be installed on the leanest cylinder.